If Gardner were trully that popular, he would have continued on in the Green Lantern title. Just face it the sales of the comic were horrible back then and they had to do someting to make a change. Having a hero become a villian is not something new, but in fact a part of great storytelling that goes back to epics like Gilgamesh, Hercules, Beowulf, and even to modern day classics, like Star Wars (and Darth Vader). Calling what they did to Hal Jordan unacceptable and rediculous is childish and we need to get over that.
As far as I can tell, sales weren't a problem with any of the titles. According to Cully Hamner, Mosaic wasn't cancelled because of sales, but because it didn't fit into the overall "editorial direction". And, by all indications, Mosaic was the least selling of the 3 monthly GL titles at the time with the last issue selling 70,000 copies. (4 if you count the Quarterly started in 1992.) Really, Emerald Dawn in 1991 had given GL a pretty solid boost.
GL had only had one period, between the cancellation of GL/GA in 1972 and the revival in 1976, when he wasn't a regular presence on the newstand. And, heck, it was the pivot title around the Millenium event. While GL may not have been the hottest title on the market in 1994, it was in no danger of cancellation either.
If anything, DC made the move not because of sales, but because of what they perceived as potential sales as they chased the Image market. Younger, hipper, edgier was the call of the day. Potential sales that were never really realized once Emerald Twilight was over and they ended up with one solid but not spectacular seller, replacing three solid sellers, in short order. It's no coincidence that not only is Hal Jordan back, the Corp and supporting characters are back too.
Getting back around to a more general point, in many respects Wally West and Kyle Rayner have benefitted and been hurt by modern storytelling. They've had an internal character arc that has been completed. Wally West has gone from immature hero with some self doubts about fulfilling his uncle's legacy, to mature, confident adult who's found the love of his life. He's had a nice 20 year plus run completing that journey, but where do you go from there? Kyle was thrust in way over his head, and grew to earning the title of Green Lantern. They're terrific characters, but perhaps not great archetypes. Neither is fighting a neverending battle for truth and justice or warring against crime and chaos. By actually telling their stories to their logical outcome, the question of what next has arisen. It's why having Peter Parker win the lottery and come to terms with the idea that he doesn't have to accept so much responsibility would be pretty much a dead end for the character. Or Bruce Banner putting his internal demons to rest permanently. Or Oliver Queen getting over his sense of adventure and becoming a responsible, sensible adult with more moderate ideals.